1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine ring segment with a cooling circuit.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as a large frame heavy duty industrial gas turbine (IGT) engine, includes a turbine with one or more rows of stator vanes and rotor blades that react with a hot gas stream from a combustor to produce mechanical work. The stator vanes guide the hot gas stream into the adjacent and downstream row of rotor blades. The first stage vanes and blades are exposed to the highest gas stream temperatures and therefore require the most amount of cooling.
The efficiency of the engine can be increased by using a higher turbine inlet temperature. However, increasing the temperature requires better cooling of the airfoils or improved materials that can withstand these higher temperatures. Turbine airfoils (vanes and blades) are cooled using a combination of convection and impingement cooling within the airfoils and film cooling on the external airfoil surfaces.
A blade outer air seal (BOAS) is formed around the turbine rotor blades 16 to create a seal against hot gas flow leakage. The BOAS is formed from a number or ring segments that together form a full annular ring around the stage of rotor blades. FIG. 1 shows a prior art ring segment with a blade ring carrier 11, a cooling air supply hole 12 formed in the ring carrier 11, two isolation rings 13 that are also formed as segments, a ring segment 15 supported by the two isolation rings 13, and an impingement plate 18 secured to the ring segment. An upstream vane 14 (left side) and a downstream vane 14 (right side) is located on both sides of the rotor blade 16 and two isolation rings 13. The impingement plate 18 includes a number of metering and impingement holes to discharge cooling air from a cooling air supply cavity 17 formed between the isolation rings 13 and the blade ring carrier 11 to a backside surface of the ring segment 15 for impingement cooling. The spent impingement cooling air is collected in an impingement pocket 19 to be discharged through cooling holes formed in the ring segment 15.
FIG. 2 shows a detailed view of a prior art ring segment with a cooling circuit. An impingement plate 18 is secured over the ring segment 15 to form an impingement cavity between the two pieces. The ring segment 15 includes cooling air holes 8 that connect to the impingement cavity 19 and discharge the spent impingement air from the cavity 19 and onto the sides of the ring segment 15 for cooling and sealing purposes. FIG. 3 shows a top view of the ring segment 15 with a number of hooks 20 that are used to secure the ring segment 15 to the isolation rings 13. The impingement cavity 19 is located between the four sides with two mate faces on the left side and the right side, and the L/E on the top and the T/E on the bottom. FIG. 4 shows a detailed view from the top of the ring segment with the cooling holes 8 connected to the cavity 19 and opening onto the four sides of the ring segment 15 to provide cooling and sealing all around the four sides.
The prior art ring segments are cooled using backside impingement cooling in the middle of the ring segment, and then using the spent impingement cooling air to cooling around the peripheral of the ring segment with the discharged cooling air then used for sealing around the sides or as purge air for adjacent cavities to prevent ingestion of the hot gas flow passing through the turbine. The discharge cooling air holes are drilled around the ring segment impingement cavity from both of the two mate faces as well as on the L/E and T/E sides. In general, the overall cooling for this circuit is very low, especially around the peripheral sides.
One issue with the prior art ring segment cooling designs is the impingement cavity supplies all of the cooling air for the peripheral cooling holes while the ring segment is subject to several circumferential and axial external gas side pressure variations. In addition, the impingement cavity pressure has to be high enough in order to satisfy any back flow margin (prevent external hot gas from flowing through the cooling holes and into the inside of the ring segment) for the ring segment leading edge. This requires a higher cooling supply pressure to prevent back flow which then leads to higher leakage flow around the ring segment. The high post impingement also induces a high pressure ratio across the ring segment trailing edge. Fewer convection cooling holes can be used at the trailing edge section for the cooling and yields a wider spacing between adjacent cooling holes.
The ring segments in an IGT engine are especially prone to early erosion due to the high gas flow temperatures that react around the segments. Ring segments typically use a TBC to provide additional protection from the high temperature gas flow. Because of transients from stopping and starting the engine, the ring segments pass through large temperature differences from the hot steady state to the cold ambient state when the engine is not running. These large temperature differences create large thermal gradients in the ring segments—as well as other parts of the turbine—that cause spalling of the TBC. Therefore, improved cooling of the ring segments is required so that part life, and therefore engine life, can be increased. Long part life is more important in an IGT engine because these engines typically operate continuously for very long periods of time, such as over 40,000 hours. Damaged parts will decrease the efficiency of the engine.